The development and adaption of optical techniques for application to communications, data processing, and similar signal transmission systems has numerous advantages as compared with electronic systems directed to comparable objectives.
Low loss, high quality, multi-mode fiber optic cable has recently been developed which renders optical communication, data processing and signal transmission systems feasible, practical, and desirable. Some of the more important advantages of fiber optic cables used in such systems include greatly reduced size, weight, and cost; electrical isolation which eliminates grounding and ground loops; reduced constraints related to impedance matching requirements; and ease of coupling to common logic circuitry by use of available light sources and detectors. Moreover, fiber optic cables when used in such systems permit high channel-to-channel isolation, easy interchangeability with electrical cable, and greatly reduced electromagnetic interference.
One of the principal advantages of fiber optic cables for use in military systems is their virtual immunity to radio frequency signals and their characteristic of being capable of containing transmitted signals wholly within the cables. In ground communications systems this attribute greatly increases transmission security. The use of optical cables also eliminates cross-talk and the optical paths provided by such fiber optic cables are resistant to electromagnetic interference from other equipment such as may be present in an aircraft, vessel, or vehicle.
In such optical systems there is a need for frequency selective optical couplers which when connected to fiber optic cables, for example, are capable of selectively extracting signal information from an optical path while rejecting or inhibiting the coupling of all other frequencies.